Problem - dark Border around Render Frame with Eevee Reflection Plane
Solution!
Render Settings > Film  set Overscan to Active

It seems that Blender trimms the area outside the camera frame and dosen't consider this informations. That's the origin of the problem. A falloff starts form there. The overscan option let some information outsite the camera frame and the effect dosen't occure.

Why this musst be like that - I don't know.